diff --git a/flake.nix b/flake.nix
index 3745050..693079a 100644
--- a/flake.nix
+++ b/flake.nix
@@ -35,9 +35,10 @@
           pkgs = import nixpkgs {
             inherit system overlays;
           };
-          rustToolchain = (pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml).override {
+          rustToolchain = (pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml).override (old: {
             targets = [ "wasm32-unknown-unknown" ];
-          };
+            extensions = old.extensions ++ [ "rust-src" ]; # Add rust-src for LSP.
+          });
         in
         {
           default = pkgs.mkShell {